1. Field of the Invention
The present invention relates to an ablation apparatus for ablating an object by laser beam, more particularly, relates to an ablation apparatus for controlling ablation of a surface of a cornea by laser beam to correct the curvature of the cornea and/or for removing the regional lesion portion thereof.
2. Description of Related Art
Recently, some methods have been proposed for correcting the refraction of an eye by ablating the surface of the cornea by laser beam (generally an excimer laser having an wavelength 193 nm is employed, also YAG laser and the like is tried to be used.) to change the curvature of the cornea. In these methods, it is necessary to control the depth of the ablation area so that it is uniform. This has been accomplished by controlling the intensity distribution of the laser beam being used for ablation such that it is constant.
For example, U.S. patent application Ser. No. 07/812,819 filed on Dec. 24, 1991, based on Japanese Patent Application No. HEI 2-416767, proposes the method for controlling the depth of the ablation by employing an excimer laser beam having an uniform beam intensity in one direction and a non-uniform beam intensity of Gaussian distribution in the vertical direction, and by scanning the laser beam into the Gaussian distribution direction by using optical elements.
In this kind of ablation mechanism, the refraction of the cornea such as myopia or astigmatism is corrected by restricting the irradiation area by employing a variable circular diaphragm or a variable slit diaphragm or the like, or by employing a special filter for changing a character of the beam transmission factor.
The operation method of removing the regional lesion portion of the cornea partially is known. This operation has a special character in restriction method of the irradiation area or positioning between the optical axis and the examinee's eye.
In the above mentioned ablation, correcting the refraction of the cornea or the removal of regional lesion portion thereof may be effectively operated. However, the boundary between the ablation area and the non-ablation area is appeared in a condition of making a clear height-difference. Therefore, not only is there a problem in the appearance, but also some dangers in that the opacity of the cornea or the like occurs without making the epithelium regeneration smoothly.
According to ablation which strips the corneal epithelium wider more than the optical zone after the operation, the epithelium regeneration proceeds from the outside of the optical zone toward the inside thereof. But if there are extreme changes of the curvature at the boundary area between the ablation area and the non-ablation area, epithelium regeneration is affected badly.